Add support for loading trunk savegames versions 293 - 299 (12.0)

Use modified upstream saveload implementation for these versions
Re-arrange headers to support multiple saveload implementations

src/order_backup.h

@@ -15,7 +15,7 @@
 #include "tile_type.h"
 #include "vehicle_type.h"
 #include "base_consist.h"
-#include "saveload/saveload.h"
+#include "saveload/saveload_common.h"
 
 /** Unique identifier for an order backup. */
 typedef uint8 OrderBackupID;
@@ -29,6 +29,11 @@ extern OrderBackupPool _order_backup_pool;
 /** Flag to pass to the vehicle construction command when an order should be preserved. */
 static const uint32 MAKE_ORDER_BACKUP_FLAG = 1U << 31;
 
+namespace upstream_sl {
+	SaveLoadTable GetOrderBackupDescription();
+	struct BKORChunkHandler;
+}
+
 /**
  * Data for backing up an order of a vehicle so it can be
  * restored after a vehicle is rebuilt in the same depot.
@@ -36,7 +41,9 @@ static const uint32 MAKE_ORDER_BACKUP_FLAG = 1U << 31;
 struct OrderBackup : OrderBackupPool::PoolItem<&_order_backup_pool>, BaseConsist {
 private:
 	friend SaveLoadTable GetOrderBackupDescription(); ///< Saving and loading of order backups.
+	friend upstream_sl::SaveLoadTable upstream_sl::GetOrderBackupDescription(); ///< Saving and loading of order backups.
 	friend void Load_BKOR();   ///< Creating empty orders upon savegame loading.
+	friend upstream_sl::BKORChunkHandler;
 	uint32 user;               ///< The user that requested the backup.
 	TileIndex tile;            ///< Tile of the depot where the order was changed.
 	GroupID group;             ///< The group the vehicle was part of.